The present invention relates generally to testing of random access memory (RAM), and more particularly to RAM testing while in operation.
Modern computer systems come equipped with a central processing unit (CPU) for performing operations on data supplied from various sources including input/output ports, volatile memory units, non-volatile memory units and other. In performing its work the CPU directly or indirectly reads and writes data to the various memory units. To expedite data processing the CPU takes advantage of a random access memory (RAM) because reading data from and writing data to a RAM is the fastest. Thus, in a typical computer system the CPU may read some of the data required for processing, e.g., an application program, from a read-only memory (ROM), but in executing the application it will store the required data in a RAM and communicate with it rather then with the ROM.
FIG. 1 illustrates a typical volatile RAM 10 with a number of physical memory pages P1 through Pn. RAM 10 is usually located on a memory card or another medium affording rapid data access rates. If a ROM is available, the application program is usually resident in the ROM. If a disk is the non-volatile medium, then the pages of the application code are loaded into RAM 10 from the disk as needed. In FIG. 1 an application program 12 is resident on a ROM 14, and the first four pages of code of application program 12 are loaded into pages P1 through P4 of RAM 10. The remainder of application program 12 does not need to be accessed for execution at this time and is thus not loaded from ROM 14. Three non-consecutive pages of a test data file 16 are uploaded into pages Pi through Pk from a disk memory unit 18. One page of a second application program 20 is loaded into page P1 from another non-volatile permanent storage unit 22. Still additional data can be loaded into remaining pages of RAM 10. A CPU can now obtain data directly from RAM 10 to run, for example, application programs 12, 20 on test data 16.
FIG. 2 shows a portion of a computer system 24 in which a CPU 26 utilizes the data stored in a RAM 28 and extends the size of RAM 28 with the aid of a virtual memory 30. Virtual memory 30 is resident on a medium offering slower data access rates than RAM 28. For example, virtual memory 30 corresponds to storage memory on a hard disk drive 40. System 24 has a memory management unit (MMU) 32, which is in charge of performing a virtual to physical mapping to keep track of which pages of virtual memory 30 are mapped to which pages of physical memory or RAM 28. MMU 32 is connected to CPU 26. CPU 26 also has a direct data bus 36 connecting it to RAM 28.
In system 24 applications and data are placed in virtual memory 30. MMU 32 maintains a table 38 to keep track of which virtual memory page Vj was assigned to which physical memory page Pi of RAM 28. As RAM 28 becomes full, certain pages which are not being used can be placed back in long term memory, e.g., on hard disk drive 40 as indicated in dashed lines or in some other long term storage medium. It should be noted that typically virtual memory 30 is considerably larger than RAM 28. For more specifics about MMUs and general memory control architecture the reader is referred to U.S. Pat. No. 4,899,275 to Sachs et al. and references cited therein.
Though infrequently, RAM bits can fail and hence it is desirable to test the memory pages of RAM. Typically, computer systems test their RAM at start-up by applying various types of tests before the computer system commences normal operation and the RAM is not otherwise in use. The test is performed because RAM, as apparent from the above discussion and for other reasons, is a critical component of the computer. In most systems a program executed from the non-volatile storage, e.g., the ROM, performs these tests. For information about suitable RAM tests the reader is referred to U.S. Pat. No. 5,923,836 to Barch et al.
Certain computer systems, however, are intended to operate without interruption, i.e., once started up they are not turned off and re-started. For example, embedded control computers of the type found in communication networks are never turned off and have to operate for long periods of time, e.g., several years. Therefore, it is not feasible to test such computer systems only at start-up with a program executed from a ROM. Instead, computer systems of this type require in-use testing of their RAM while there is live data in the RAM. At present no suitable techniques for performing such in-service RAM testing exist.
It is therefore a primary object of the present invention to provide a method for in-service testing of RAM. Specifically, the method of the invention is intended for use in computer systems designed to operate without interruption.
It is another object of the invention is to provide a simple in-service RAM testing method which can be easily implemented on any computer system requiring such testing, such as, for example, a computer system used in a communication network, such as an optical transport system.
These and other objects and advantages of the invention will become apparent upon further reading of the specification.
The objects and advantages are achieved by a method for in-service testing of a random access memory (RAM) of a computer system having a virtual memory. The method calls for identifying a number n of physical memory units pi, i=1 . . . n, in the RAM and a number nxe2x88x921 of virtual memory units vj, j=1 . . . nxe2x88x921, in the virtual memory. A one-to-one mapping is performed to map the physical memory units pi to virtual memory units vj such that a physical memory unit px is left unmapped. The data is then tested in the unmapped physical memory unit px. The testing methods used to verify the data can involve destructive data tests or non-destructive data tests. After the data in physical memory unit px is tested, data from another physical memory unit py is copied to physical memory unit px and a one-to-one re-mapping of physical memory units pi to virtual memory units vj is performed such that physical memory unit py is left unmapped. The data in physical memory unit py is then tested. Clearly, the steps of copying, one-to-one re-mapping and testing can and are preferably repeated until all physical memory units pi are tested.
The method of the invention can be applied in a sequential manner. In other words, the one-to-one mapping can be sequential where physical memory unit px is left out of the sequence and physical memory unit py selected for testing after physical memory unit px is tested is adjacent physical memory unit px such that py=pxxc2x11.
It is important to write-protect physical memory unit py after data from it is copied to physical memory unit px to thus prevent unwanted over-writing. Should a write request to write a new data to physical memory unit py be received, then the writing should be enabled in order not to interfere with the regular in-service operation of the computer system and the new data should be written to physical memory unit py. However, after the new data is written it needs to also be placed in physical memory unit px. This can be done by either re-copying the new data from physical memory unit py to physical memory unit px or by writing the new data to physical memory unit px.
Preferably, the testing method of the invention is applied at the level of memory pages. In other words, the physical and virtual memory units are entire memory pages. Furthermore, when any defective memory units or pages are found a remedy, e.g., an error correction algorithm, can be applied to that page. Alternatively, the defective memory unit or page can be flagged and taken out of service.
Advantageously, a memory management unit (MMU) is used for performing the one-to-one mapping and the one-to-one re-mapping operations. The MMU can be placed in charge of the copy requests as well and thus be an instrumental part of the testing process in accordance with the invention.
The virtual memory units or pages used in the testing process can be located in a non-volatile memory, e.g., on a disk drive used by the computer system. Depending on the type of computer system and its lifespan, the testing should be performed at defined times. For example, the testing can be performed cyclically after certain time intervals, e.g., every minute or every day. Alternatively, the testing can be performed at a very slow rate and run continuously in the background.
The invention can be applied to a computer in a network, e.g., to a computer which is a part of a communication network. In this case the test is performed in the same manner, but the entire communication network can be alerted of the state of the physical memory units pi tested in the computer. Conveniently, when physical memory units pi are flagged for being found defective during the testing the entire network can be alerted to that fact.
The invention further encompasses a storage medium in which the steps of the method are stored. The storage medium can be any suitable non-volatile or volatile memory which can be interfaced with the computer system or is a part of the computer system.
As will be apparent to a person skilled in the art, the invention admits of a large number of embodiments and versions. The below detailed description and drawings serve to further elucidate the principles of the invention and some of its embodiments.